This invention relates to a nuclear power plant and, more particularly, to a nuclear power plant in which a treatment for suppressing deposition of radioactive substance is performed for structural material to be used while coming into contact with a liquid in which radioactive substance is dissolved, such as a piping of the primary cooling water system.
The piping, pumps, valves, or the like which are used in the primary cooling water system of a nuclear power station are made of stainless steel, Stellite, or the like. When these metals are used for a long period of time, they are subject to corrosion and damage, causing composite metal elements to be dissolved in the primary cooling water and brought into the nuclear reactor. Most of the dissolved metal elements are oxidized and deposited on the fuel rods, where they are irradiated by neutrons. Hence, the radioactive nuclides such as .sup.60 Co, .sup.58 Co, .sup.51 Cr, .sup.64 Mn, etc. are produced. These radioactive nuclides are redissolved in the primary cooling water and float as ions or insoluble solid components (hereinafter, referred to as clads). Portions of the floating clads are removed by a desalting device or the like for purification of the reactor water. However, the remaining clads are deposited on the surface of the structural material consisting mainly of stainless steel while they are circulating in the primary cooling water system. Therefore, a dose rate on the surface of the structural material increases and there is a problem on radiation exposure of workers who perform maintenance and inspection.
Thus, as a method of preventing an increase in the dose rate at the surface of the structural material, the methods of elimination of the radioactive substances deposited on the structural material are studied and carried out. Presently, the eliminating method includes the following three methods.
(1) Mechanical cleaning method
(2) Chemical cleaning method
(3) Cleaning method by electrolysis
The method (1) is mainly applied to parts, for example, the surface is cleaned by a high-pressure jet water. However, it is difficult to remove the radioactive substance having a large adhesive property by this method, and is impossible to systematically clean over a wide range. In fact, even if the dose rate were reduced temporarily by this method, the dose rate tends to increase again due to the subsequent use over a long period of time.
According to the method (2), the oxide film on the surface of the steel is dissolved by the chemical reaction using chemicals such as acid solution or the like, thereby removing the radioactive substance existing in the oxide film. This method has a problem of corrosive damage of the structural material due to the chemicals. That is to say, when the oxide film is dissolved, the structural material is also subject to corrosive damage and there is a fear of causing a stress corrosion cracking of the structural material due to the small amount of chemicals remaining after the cleaning.
The method (3) has also the same problem as the method (1).
Furthermore, to reduce the quantity of adhered radioactive substance, it is performed a method of suppressing elution of metal elements which are the sources of radioactive substance. Namely, oxygen or hydrogen peroxide is injected into the water supplying system to suppress the corrosion of the structural material, thereby reducing the amount of corrosive products entering into the nuclear reactor.
However, even if such a method is used, it is impossible to completely prevent the corrosion of the structural material of the primary cooling water system and to eliminate the radioactive substances in the primary cooling water; therefore, a problem on increase in dose rate at the surface due to the deposition of the radioactive substance into the structural material still remains.